[LoopVectorize] NFC: Change computeFeasibleMaxVF to operate on ElementCount.

This patch is NFC and changes occurrences of `unsigned MaxVectorSize`
to work on type ElementCount.

This patch is a preparatory patch with the ultimate goal of making
`computeMaxVF()` return both a max fixed VF and a max scalable VF,
so that `selectVectorizationFactor()` can pick the most cost-effective
vectorization factor.

Reviewed By: kmclaughlin

Differential Revision: https://reviews.llvm.org/D96018